Add terms stats to the terms panel.

This pr allow user to compute values in term panel using the following modes : 
* count
* total
* min 
* max
* mean

When you select other mode than count : 
- A field "Value field" appear so you can choose on which field the computation is performed. 
- The "Exclude Terms(s)" field is hidden, because terms stats is not
- The sort field will contain more options : total, reverse_total, min, reverse_min, max, reverse_max, mean, reverse_mean

Please let me know if something should be updated.